It has heretofore been a conventional practice to build a computer main memory by use of a plurality of memory units. The address for a particular location in the main memory includes a section that designates the particular unit and a further portion that designates a particular storage location on the card for the unit.
A typical memory card comprises an array of memory chips which are typically dynamic random access memory circuits. Dynamic access memory chips have undergone a steady progression in capacity through the years. During each period of several years the capacity of memory chips typically increases by a factor of four. Although a memory system designer is aware that greater capacity memory chips may be available in the future, the designer must finalize his design with a selected type of memory chip. The overall system must then be designed to operate with the selected memory chip.
The demand for greater memory capacity in computer systems appears to be insatiable. When new memory chips become available at competitive prices it is often the desire of computer system users to upgrade the capacity of their systems by using the new chips even though the existing system was designed around the previous generation of memory chip. To incorporate the increased memory it is necessary to not only replace all of the existing memory cards but to substantially redesign the memory control unit of the computer and possibly the central processor to accommodate the increased capacity and the new distribution of memory capacity on the improved memory cards. The requirement to not only replace all of the memory cards but also to redesign a substantial part of the system can entail substantial costs as well as possible loss of operating time for the system.
In view of the above problems and the anticipated requirement for increasing the memory size of computer systems, there exists a need for a design for a memory which can permit the intermixing of different capacity memory units within a main memory or the total changeover to greater capacity memory cards without the need for system redesign and hardware replacement.